Single chip photoresponsive devices are useful for vehicular applications, including occupant detection, vehicle guidance and collision avoidance. Generally, in both automotive and non-automotive applications, photoresponsive devices have been implemented to provide information that is subsequently displayed to the operator of the vehicle or machine. In such applications, providing information in a manner compatible with the human visual system is important to optimize the transfer of information to the operator. For automotive systems, however, the need to display the information is of less importance than the need to electronically analyze the data. Thus, requirements for an automotive photoresponsive device diverge from those of mainstream commercial photoresponsive devices.
Most known imagers integrate active transistors directly into the imager chip. The degree of integration varies depending on both the application and upon the process technology. Imagers that can be formed using a conventional Complementary Metal Oxide Semiconductor (CMOS) process offer the greatest opportunity for peripheral electronics integration and cost reduction.
In a typical CMOS imager, light penetrates various transparent insulating films deposited upon the surface of the wafer for electrical and mechanical protection, and is absorbed in an active device, generally in the form of a photodiode or photocapacitor. It is important to note that the absorption coefficient of light in silicon is inversely proportional to the wavelength of the light. Thus, blue light, which has a shorter wavelength, is absorbed at relatively shallow depths, as compared to red light, which has a longer wavelength and consequently is absorbed more deeply in the silicon.
When a photon strikes a semiconductor it can promote an electron from the valence band to the conduction band creating an electron/hole pair. In order for the absorption of light to be detected in the photoresponsive device, the electron hole pairs that are produced must be separated before they can recombine. This separation is generally accomplished by the application of an electric field, either in the form of a photodiode structure or in the form of a photocapacitor or phototransistor structure. The electric field produces a depletion region in the affected semiconductor region and any free charges in this region are rapidly swept away. Significantly, positive charges are driven in one direction while negative charges are driven in the opposite direction. Consequently, electron/hole pairs are separated before they can recombine and can be detected by external means, and the presence of the original photon can be inferred.
However, the structures required to produce the electric field cover most of the substrate. Existing alternatives use a continuous polycrystalline silicon film that forms the top electrode of the photoresponsive device structure. Generally, a higher optical absorption coefficient for blue light means that a significant amount of the shorter (blue) wavelength light is absorbed in the top layer of silicon. Light absorbed in this region is not detected. Thus, photoresponsive devices typically exhibit poor performance for shorter wavelength light.
A complementary problem is typically observed for long wavelength red and near IR photons. These photons can penetrate relatively deeply into the active charge collection material. In fact, they can penetrate sufficiently deep that adjacent devices can collect the resulting electron/hole pairs, thus reducing the resolution of the image in the long wavelength regions of the optical band.